Cooling presents a significant consideration when designing power transformers, especially for hi-frequency applications (e.g. 50 kHz to 500 kHz) which are much smaller in size relative to low frequency transformers. For example, a 15 kW, 100 kHz transformer is around 460 cc in volume, and might ideally have 1% power losses (i.e. 150 W), but more realistically, 2 to 3% losses are expected. A good cooling system is required to dissipate heat related to these losses.
The main losses of a transformer are determined by power losses of the winding, so it is very important to provide cooling of the winding. Losses are somewhat directly proportional to current, so higher currents (e.g a winding with a current of 300 A or more) present increased cooling challenges for the winding and winding terminations, including challenges relating to connecting the transformer with the power stage.
The design of the transformer described in U.S. Pat. No. 7,123,123 (the contents of which are incorporated herein by reference as if fully rewritten herein) provides good performance regarding efficiency, cooling, and integration of transformer into the power stage, but it can be used only when the high current winding has one turn.
The subject technology maintains many of the advantages of U.S. Pat. No. 7,123,123, and can additionally be used with two or more turns in the high current winding. In the various aspects of the subject technology described herein, a substantially flat and elongate conductor is used. More specifically, a high current transformer winding made from a flat conductor having opposing ends that are shaped (e.g. a lateral protrusion), such that when a middle portion of the conductor is wound around a transformer core, one or both opposing ends protrude to allow operative connection to a power source. In one aspect, “operatively connected to a power source” comprises being bolted to a bus bar. In one aspect, mounting holes (7) are disposed in conductor (20) (e.g. FIG. 3B) to allow bolting to a bus bar. In one aspect, opposing ends of a substantially flat & elongate conductor are folded so as to form lateral protrusions to allow operative connection to a power source. Such a flat conductor can be wound around a core multiple times. Conventional conductors are used for the low current, high voltage winding of the subject technology.
The flat, elongate high current winding of the subject technology (copper foil in one aspect) allows higher current capacity (e.g. twice as much), relative to a single winding configuration. Another advantage is achieved because the flat conductor allows more efficient heat transfer. The flat conductor bolted to a bus bar offers a relatively higher surface area at the junction, which mitigates heat buildup. The junction between the round and flat conductor of conventional systems creates an unwanted hot spot due to the decreased cross-sectional area and junction losses. The subject technology provides a flat terminal made out of a unitary piece of material (i.e. no junction between flat and round conductors) that can be bolted directly to the bus bar. The foregoing provides not only improved cooling, but also improved electrical conductance characteristics.
In one aspect, a 90 degree turn is imposed on each opposing end of the flat conductor in order to provide outwardly protruding end terminals that bolt directly to a bus bar. The 90 degree turn imposed on each end of flat conductor can be achieved various ways. The flat conductor can be folded on each end at a 45 degree angle, resulting in a 90 degree turn (e.g. FIG. 1B). Alternatively, the turn can be cut to shape as shown in FIG. 1A (e.g. using copper foil). The 90 degree turn can be oriented to either have both terminals project outwardly from the same or opposite sides of the core. The former configuration is needed for center tap embodiments as described elsewhere herein.
In some aspects (e.g. FIG. 6), two turns are disposed in each opposing end. Doing so results in the topology depicted in FIG. 5 wherein first and second lateral protrusions 28/29 are facing away from each other. The length of the conductor relative to outer perimeter of the core about which the conductor is wound, determines the number of turns, which consequently determines the final orientation of the end protrusions. In other words, the length of the conductor can be adjusted so that end protrusions 28/29 are facing the same way.
A conventional round conductor is wound around, and radially outside of, the high current winding to create the low current, high voltage winding. The low current conductor is wound around a bobbin in one aspect. Thus, the high and low current windings are concentrically and/or coaxially oriented with respect to each other. It is to be noted that the terms “concentric” and “coaxial” are specifically defined herein to include this relationship. The concentric high and low current windings surround a portion of a transformer core that is part of a core assembly that forms a magnetic flux circuit in one aspect.